Make shortening treshold adjustable by lib consumers

[infeo]

WIth Issue #102, the value to restrict the ciphertext filename length (maxNameLength) was splitted into two values: One for the cleartext length and one for the ciphertext length (shorteningThreshold).

The latter was moved for the time being into the internals of the library, making it not adjustable to lib consumers. Since there are use cases where a consumer want to change the default value (220), we should add this to the public api again.

[FusionX9000]

Is it possible to update shortening threshold and migrate existing vault?

[infeo]

Is it possible to update shortening threshold and migrate existing vault?

No, this is out of scope of this issue.